1. Field of Disclosure
The disclosure relates to high power fiber laser systems. In particular, the disclosure relates to a fiber provided with clad mode absorber operative to decouple undesirable light guided in a cladding.
2. Prior Art
High power fiber laser systems (“HPFLS”), are typically configured with one or more amplifying cascades each including an active fiber which is either side or end pumped by multimode (MM) pump light. The latter is not always fully absorbed and continues to propagate along the fiber's inner cladding under a polymeric protective sheath which has a lower refractive index than that of quartz. With increased powers of current pump sources, it is not unusual that this residual pump light guided in the cladding reaches kW levels. Fibers experience bends and other mechanical stresses leading to micro distortions guiding undesirable kW clad light towards the sheath that can be easily damaged.
Still another source of undesired clad light that may destroy a protective sheath includes splice regions between fibers. Spliced fibers are not always uniformly dimensioned and/or not perfectly spliced together which create favorable conditions for excitation of MM light in splice regions. This undesirable light is also guided along the cladding and adds to already high power of unabsorbed pump light. Also, high power light back reflected from the laser treated surface may be coupled in the cladding and thus contribute to high powers that may heat the protective sheath up to critical temperatures.
Furthermore, in high power SM fiber laser systems, the MM radiation guided in the cladding may damage the end regions of the fiber. Finally, the presence of the MM light at the output end of single mode delivery fibers in high power SM laser systems affects the quality of the SM signal light. Based on the foregoing, MM clad light is highly undesirable and should be removed.
Referring to FIG. 1, typically, a high power fiber laser system is configured with a passive fiber 10 located downstream from amplifying gain blocks and delivering amplified signal light to the surface to be treated by the signal light. The fiber 10 includes a core 12 guiding the signal light, a wave guiding cladding 14 supporting undesirable MM clad light and a polymeric protective sheath 16 preventing the MM light from escaping cladding 14. Optionally, delivery fiber 10 may also have an outer cladding with a refractive index lower than that one of cladding 14. As discussed above, the MM light guided along waveguide cladding 14 is highly undesirable, hazardous and therefore should be decoupled from waveguide cladding 14.
Devices configured to remove clad light and convert the light energy into heat energy are known as, among others, cladding mode absorbers or strippers (“CMA” or “CMS”) Typically, a CMS is provided along a length of one or more passive fibers, for example, a delivery fiber which is stripped from the protective sheath 18 and configured with a higher refractive index than that one of quartz, i.e. wave guiding cladding 14.
Some of structural limitations of known CMSs include a low thermal conductivity which leads to localized removal of high power clad light. The known silicone absorbers typically allow decoupling cladding light with a power varying from about 100 to about 400 W, which is considered to be not particularly damaging to silicone and/or other optical components. Yet this power range tolerance is insufficient for modern multi-kW high power fiber systems in which localized removal of powers higher than about 400 W may heat a CMS at temperatures that can easily damage the CMS and other optical components.
Also, the MM light includes both high numerical aperture (“NA”) light and low NA light. The high NA light propagates at relatively large angles and may be easily absorbed. However, low NA light propagating at angles of up to about 45° is rarely absorbed and thus continues its propagation almost unabsorbed. Reaching the system output, the unabsorbed MM light considerably worsens the quality of the output signal light.
A need therefore exists for a high power CMS used in conjunction with high power fiber laser systems and configured to maximize decoupling of MM clad light from a waveguide cladding.
Another need exists for a CMS configured to provide substantially uniformly distributed absorption of clad light along its entire length.
Yet another need exists for a high power CMS having a high-temperature resistant structure.